Backbone of contention

So, you’ve got a fast Internet connection, but you’re either not getting that speed or you’re not getting that speed to certain sites or services. You’ve got a 50Mbps connection, but some services claim that your connection is “too slow” and disconnect you. Why is that?

Well, it comes down to what are called “contention ratios”. It doesn’t matter how fast your connection is, or how fast the server’s connection to the Internet is if the data cannot be passed through the systems in between at anywhere near those speeds.

So, what’s that all about?

Internet Service Providers, whether providing residential or business grade connections, use a simple formula called a contention ratio.

It’s the Internet service equivalent of overbooking a flight – a common practice among some airlines, who know there’s a certain percentage of passengers who typically won’t show up before take-off.

For a simple example, let’s say we have one hundred broadband customers, each with a 10Mbps (ten megabits per second) Internet connection. The ISP has their own connection to the Internet along which all of that traffic flows in and out.

You might expect that connection to be at least 100×10 megabits (1000Mbps or 1Gbps, more or less), with perhaps a little extra margin for connecting new customers, right? That allows each customer to receive data at maximum speed.

And you’d be wrong.

That’s a contention ratio of 1:1, and is pretty much unheard of.

The ISP usually has a connection with less capacity than the sum of the connections sold or leased to customers.

If our hypothetical ISP has a connection of only 250Mbps, that’s a contention ratio of 4:1. One quarter of their customers can get data at full-speed if (and only if) the other three-quarters are idle. In practice, most connections are doing something most of the time, so it’s rare that anyone is going to get their full 10Mbps of data rate.

If you’re Web-browsing or doing email, or using an instant-message client, this sort of thing doesn’t matter very much. There’s many seconds between your computer sending small requests and receiving small bundles of data, so everyone gets good response times. As soon as you want a lot of data, however, it all falls down, because you’re fighting for a resource that is not sufficient for all of the customers.

A 4:1 contention ratio is pretty rare. Last time I saw one it was being offered as a “top class AAA” connection.

Most ISPs actually operate on a contention ratio of between 20:1 and 40:1; that is, their connections to the Internet are between one twentieth and one fortieth of the sum of their customer’s data-rates. Good luck getting the maximum data rates out of those connections.

I’ve even seen a few that operate at 100:1 or more! Really.

Now, remember that your ISP is connected through another provider, who is connected through another provider, and so on. The transit portions of the Internet are, essentially, just a big blob of interconnected providers. All with various contention ratios that are most definitely not 1:1. Everyone oversells their connections.

Because of the high contention ratios, big data transfers (videos, file-sharing and so on) and streams with lots of little data-packets (MMOGs like World of Warcraft, or virtual environments like Second Life) can slow response even further for folks who are just routinely fiddling about on the Web. Thus, it’s a routine practice among ISPs to ‘throttle’ or lower the priority of these sorts of services – effectively introducing lag.

So, the next time you’re wondering why your connection to a service is so darned slow, just remember, it might not be your fault or the fault of the service, but someone in the middle with a high contention ratio, who may well be slowing down your access to keep other services faster for other users.

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Categories: Internet, Technology.



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